Christ taught that those who teach others NOT to do the Law would be called least in the Kingdom, however the dietary laws and others are specifically opposed in later books (e.g Acts). How do you reconcile this seeming contradiction?
To answer your question, let's look at the full context of Jesus words in Matthew 5:
Matt. 5:17 “Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill.
Matt. 5:18 “For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished.
Matt. 5:19 “Whoever then annuls one of the least of these commandments, and teaches others to do the same, shall be called least in the kingdom of heaven; but whoever keeps and teaches them, he shall be called great in the kingdom of heaven.
Matt. 5:20 “For I say to you that unless your righteousness surpasses that of the scribes and Pharisees, you will not enter the kingdom of heaven.
Jesus is speaking to the crowd in response to the Pharisees' accusation that Jesus was trying to abolish the Law of God. Jesus refuted this accusation, saying that His purpose in appearing before men was not to abolish anything. Rather, Jesus came to fulfill the Law. The word fulfill in Greek is pleroo, which is also translated "to complete." In other words, Jesus came to complete or accomplish the entire Law.
He goes on to say that heaven and earth will not pass away until the entire Law of God has been accomplished. Jesus is the One Who accomplishes God's Law by keeping the entire Law perfectly. Jesus never sinned during His earthly life, and so as He went to His death on the cross, He had accomplished or completed all the requirements of God's Law. Jesus was referring to His completing of the Law when on the cross He said, "It is finished."
The Pharisees were accusing Jesus of setting the Law aside, while they endeavored to keep it in their own power. Jesus condemned the Pharisees and anyone else who tried to follow their example by saying that unless they could keep the Law even better than the Pharisees kept it, they could not enter the Kingdom of Heaven. This was a daunting challenge, because the Pharisees were scrupulous followers of the Law. If the Pharisees didn't have enough righteousness on their own to enter Heaven, then who could?
Obviously, Jesus was alluding to that answer. No one can earn their way into Heaven, because no one is righteous enough to make it through works. This is precisely why Jesus came to fulfill the Law on our behalf, so that His perfect work could be credited to us on the basis of faith.
Now that the Law has been completed by Jesus on our behalf, we are no longer obligated to keep it ourselves. That is not to suggest that the Law itself has gone away, but only that our obligations to keep it is gone, because Jesus has already kept (i.e., fulfilled, completed, accomplished) it for us. Likewise, anyone who discounts the importance of God's Word will suffer loss.
Jesus is the One Who kept and taught the Law, and as a result, Jesus will be the One Who is called great in the Kingdom of Heaven (Matt 5:19). We should not teach believers to follow the Law out of obligation, but neither should we teach that the Law is no longer in force. Rather, we teach that the Law has been fulfilled by Christ's work, and we rest in His perfect accomplishment of the Law rather than in our own futile efforts to keep it.